1. The Field of the Invention
The invention relates in general to dual copy data storage systems and more particularly to the read-only recovery of a logical volume using a selective dual copy within a virtual tape server (VTS) system in a manner that is transparent to and asynchronous with a host.
2. The Relevant Art
High density, removable media storage libraries are used to provide large quantities of storage in networked computer systems. Typically, such data storage systems are employed for backup or other secondary storage purposes, but the data storage system may also be used as primary storage in circumstances that are conducive to sequential data access and the like.
The data is stored on media cartridges, such as magnetic tapes or optical disks, that are arranged in storage bins and accessed when data on a cartridge is requested. Currently available media cartridges are capable of storing much more data than the data volume units that correspond to the size of early types of media cartridges. For example, a data volume that corresponds to a 400 megabyte disk may now be stored on a disk with up to 60 gigabytes of storage capacity. Unfortunately, much legacy equipment in existing computer systems is configured for the smaller volume sizes.
Volume mapping is used to create a correlation between the physical capacity of a storage cartridge (stack volume or physical volume) and the data storage unit size (virtual volume or logical volume) of a file or block that is stored on the cartridge. Given the available data storage capacity of a single storage media cartridge, such mapping allows multiple logical volumes to be stored on a single physical volume, hence providing an efficient use of the available storage media. A virtual tape server (VTS) is one device capable of creating and maintaining such mapping among physical volumes and logical volumes.
A typical VTS system includes a virtual tape server and an automated media library. The library is controlled by a library manager that is similar to a workstation computer. Within the VTS system, typically two databases reside on separate memory disks within the system. One database resides on the virtual tape server and the other resides within the library manager.
The virtual tape server database contains the logical-to-physical volume mapping, as well as information concerning volume attributes that define actions that have been or will be taken on a logical volume each time it is closed. One of the attributes included in such information is whether a secondary copy of a logical volume was made when it was last written.
The library manager database also contains attributes associated with the logical volumes stored on the media cartridges. Included in these attributes are the construct names and associated attributes corresponding to each logical volume in the library. The library manager also controls the physical loading of media cartridges in corresponding drives by storing the physical location of the physical volumes within the storage bins and controlling a robotic accessor arm that retrieves the physical volumes from the bins and load the cartridges in the drives when a mount request is received.
Through proper communication of the volume mapping and construct attributes, a host processor and peripheral data storage equipment may access logical volumes as though they were individual physical volumes. The volume access management is provided via the virtual tape server and library manager as described above.
In certain circumstances, it may be desirable to make two copies of a single logical volume. It may also be desirable to store such copies on multiple physical volumes, such as on separate cartridges or even in separate geographic locations, so as to avoid loss due to failure of a single tape or tape drive unit. Additionally, it is desirable to provide a method of recovering the secondary copy of the logical volume if, for some reason, the primary copy becomes unavailable. Some systems and methods have been proposed to recover a secondary copy of a logical volume in a dual copy storage system. Unfortunately, a number of deficiencies exist in such known systems and methods.
For example, many known dual copy systems require an explicit command from the host to initiate recovery of a secondary copy. The software running on the host must be modified to provide the recovery command. As a result, such a recovery system may be difficult to use with existing (legacy) software. Consequently, adding recovery capability to an existing system may be rather difficult. Prior art recovery systems may also require that the host transmit the data to the virtual tape server repeatedly in order to recover the secondary copy. Thus, the I/O resources of the host are unduly taxed.
Furthermore, some dual copy systems require the host to track the locations, i.e., the physical media cartridges, of the logical volumes. This may require the maintenance of a special database on the host to hold meta data for each file stored in the virtual tape server. Again, such recovery systems are difficult or impossible to incorporate into existing host systems without providing new host software, and may unduly tax the resources of the host. Additionally, such systems may be difficult to use in a heterogeneous environment, i.e., with host computers that use different operating systems, file formats, etc.
Thus, it would be an advancement in the art to provide a virtual tape system capable of recovering a secondary copy of a logical volume in a manner that is substantially transparent to the host. It would further be an advancement in the art to provide a virtual tape system capable of efficiently recovering a logical volume independent of the host. Yet further, it would be an advancement in the art to provide a virtual tape system that minimizes the virtual tape server resources required to recover the secondary copy.